Advances in the field of electronics have led to a reduction in both core and input-output power supplies in order to increase speed and reduce power consumption. The transistor dimensions and oxide thickness have also decreased for the same speed reasons.
Thus, for devices of 45 nm, the “standard” external power is now 1.8V where it was previously 3.3V or 2.5V. In order to be able to reach high frequencies the oxide thickness has been decreased and is now around 28 to 32 Å for 1.8V devices (where previously it was about 50 Å).
There are now some apparatus that operate with these new smaller more fragile devices in circuits that have to handle signals suitable for the old higher voltage domains. For example, to be compatible with some older chips and a few standard protocols some input-output cells must operate at a nominal 3.3V to generate signals at this level using the new devices that operate at a nominal 1.8V.
In order to ensure that these devices are not overstressed with the accompanying problems of oxide breakdown and lifetime degradation due to HCI (hot carrier injection) precautions need to be taken.
One way of protecting the devices is to provide an intermediate voltage supply, such that the higher voltage domain is divided in two. This intermediate voltage supply may be the high voltage supply of the lower power domain of the new devices, in this example 1.8V. The devices can then be arranged either between this intermediate voltage level and the high voltage level of the higher voltage domain, in this case 3.3 V, or between the intermediate voltage level and ground. In this way the devices only see half of the 3.3V across them and they are protected. However, care must be taken on power up that the 3.3V does not switch on before the 1.8V intermediate level as if it did some devices would see the full 3.3V and be overstressed. Thus, these circuits are set up so that the intermediate voltage is always switched on before the high voltage.
There can be additional problems that arise due to the intermediate voltage level being switched on before the high voltage level signal and the present invention seeks to address these problems.
Care must also be taken for devices that actually receive or output the high level signals that they too are not overstressed.